High quality golf club sets are produced and marketed in what is termed "frequency matched sets", each golf club being constructed such that the flexing characteristics of the club will provide the same degree of "feel" throughout the set. Although "feel" is somewhat subjective, it is generally well accepted that a golf club which provides proper "feel" will aid the golfer in achieving: (i) optimum club head velocity and club head position at the point of ball impact--providing better overall shots; and (ii) greater uniformity from shot to shot--both of which will contribute to lower total scores. U.S. Pat. No. 4,070,022, the disclosure of which is incorporated herein by reference, is directed to a method for accurately quantifying relative "feel", based on accurate determinations of the frequency of vibration of a specific shaft. After the frequency determinations are made, shafts are selected from a plurality of selected shafts in which the frequencies fall on a predetermined gradient formed by a plot of shaft frequency versus shaft length, in which shaft frequency increases as shaft length decreases. Subsequent mating of the shafts with weight-matched club heads, i.e., wood and iron heads, produces a set of matched golf clubs.
The utility of the method described in the '022 patent is, in part, based on the finding that frequency measurements of various shafts can be reproducible and therefore serve as a reliable index of shaft flexibility. Frequency measurement is generally accomplished by securing the butt end of the shaft in a clamp or chuck. A predetermined test weight is fixed to the tip end of the shaft, after which the shaft is plucked so as to cause it to vibrate. Reproducibility of such vibrating frequency is achieved by depressing the tip end to a predetermined stop (i.e., such that each shaft will have the same amplitude of vibration) and thereafter releasing the shaft such that the resulting oscillations may be measured utilizing an electronic counter unit. Utilizing this system, reproducibility of measurements of +0.2 cpm can be realized--at least with respect to the high quality steel shafts presently available.
It was found, however, when the same method was employed for the frequency measurement of composite (generally graphite) shafts, that reproducibility was poor or non-existent. Composite shafts are made of fiber, e.g., graphite, reinforced resin. The shafts are made by cross lapping various plies of reinforced fibers which have been impregnated with a resin. A cylindrical steel mandrel, which has been precoated with a release agent, is then rolled between flat planes--such that the resin-impregnated, woven fabrics are rolled upon the mandrel and upon the fabric itself a number of times. After the multiple plies are wrapped around the mandrel to achieve the desired diameter, the entire unit is wrapped to maintain the plies tightly wrapped during the subsequent curing procedure. It is therefore readily seen, unless special precautions are taken, that the resulting composite shaft will not be completely uniform in cross section. This cross sectional non-uniformity results in a tube in which the flex (frequency) will vary along different lines of the shaft, parallel to the longitudinal axis of the shaft. Various manufacturers of shafts have labeled their product as "frequency matched". While there is no industry-wide standard, that term is generally understood to define a set of clubs in which a plot of shaft frequency, "f", versus shaft length, "1", will fall on essentially a straight line (i.e., f=ml+b) with a variation not exceeding .+-.1.0%, preferably not exceeding +1 cpm. The graphite products that are presently marketed exhibit far greater discrepancies in frequency.